1. Technical Field
The present invention relates to broadcasting RF signals and, more particularly, to improvements in high level combining of analog FM and digital signals.
2. Description of the Prior Art
It has been known in the radio broadcasting art to transmit a composite signal consisting of a frequency modulated (FM) analog signal and a digital signal, such as an in band on channel (IBOC) digital signal. The prior art includes, for example, the U.S. patents to Murphy et al. U.S. Pat. No. 5,315,583 and Papadopoulos et al. U.S. Pat. No. 6,144,705.
The prior art also includes separate amplification of such signals, sometimes referred to as IBOC high-level combining. An example is illustrated in FIG. 1 herein. In this example, the output ν2 of an existing FM transmitter 10, illustrated as a main FM transmitter, is combined with the output ν3 from a digital transmitter referred to as a digital TX (linear) transmitter. The input to the main transmitter 10 is obtained from a conventional FM signal source 14 while the input to the digital transmitter 12 is obtained from a conventional IBOC source 16. The outputs ν2 and ν3 of these transmitters are combined in a conventional coupler C, which typically is a 10 dB coupler. The coupler, which is sometimes known as an output coupler, provides an output ν1, that is supplied to a transmitting antenna 20 for broadcasting the composite signal. The coupler C has ports 1, 2, 3 and 4 with the voltages ν2 and ν3 being applied to ports 2 and 3. The output at port 1 is supplied to the antenna 20. Port 4 is coupled to a reject load RL.
Because the outputs ν1 and ν2 are combined only after they have reached a high level of amplitude (because they have already been amplified by separate amplifiers) this is referred to in the art as “high-level combining” or “separate amplification”. This type of combining results in high losses. This may be viewed as the penalty paid for the simplicity involved. In a 10 dB coupler, the problems that are noted include the following: the main FM transmitter needs to have enough headroom in order to increase its output power by 11% to overcome the combiner insertion loss. This can be very problematic in specific installations without additional headroom to spare. Major hardware upgrade could be necessary to overcome this issue, such as by replacing the existing main FM transmitter with a more powerful transmitter. A second problem with this type of system is that the overall dissipation increases. Besides the power dissipated by digital transmitter 12, additional energy is wasted at this reject load RL where up to 10% of the main transmitter FM output and up to 90% of the output of the digital transmitter will be dissipated. This inefficiency creates additional heat load for the air-conditioning equipment in the room containing the transmitter equipment.
The present invention is directed toward improvements in the combining of an output of an FM transmitter with that of a digital transmitter. The improvements are directed toward minimizing the FM and digital power losses in high level combining. The improved system provides various benefits including higher overall system efficiency and elimination of requirements for FM headroom.